Session-aware binding event listeners
* Session object is stored in a domain can have multiple states:
"Bound to the Session; domain unbound from the Session; (1) with the Session object persistence to a storage device; (2) with a Session object from the storage device to restore .
* Servlet defines two interfaces help JavaBean objects understand their domain of these states Session :
HttpSessionBindingListener interfaces and interfaces HttpSessionActivationListener
Web.xml files do not need to register the class implement both interfaces in
HttpSessionBingdingListener接口
* Achieved JavaBean objects HttpSessionBingdingListener interface may perceive themselves to be bound to the Session events in and removed from the Session
* When an object is bound to the HttpSession object, web server calls the object's void valueBound (HttpSessionBindingEvent event) Method
* When an object unbound from the HttpSession object, web server calls the object's void valueUnbound (HttpSessionBindingEvent event) method
HttpSessionBindingEvent：
getName()
getValue()
getSession()
The listener is invoked less